Why is it so cold in Tomb-Sweeping Day?

It often rains around Tomb-Sweeping Day every year, and this year is no exception. From April 5 to this weekend, most of the south will be covered with rainy weather. Therefore, Du Mu, a poet in the Tang Dynasty, said in a poem: "There are many rains during the Qingming Festival, and pedestrians on the road want to break their souls." . Why does it always rain in the south of Tomb-Sweeping Day? First, when winter goes and spring comes, the cold air gradually weakens, and the warm and humid air on the ocean begins to move northward actively. Before and after Tomb-Sweeping Day, cold and warm air often meet in the south of the Yangtze River, resulting in rainy weather. The second is the spring in Jiangnan, with a lot of low pressure. The clouds in the low pressure move fast, the wind is strong and the rain is urgent. Whenever the low pressure passes, there will be rainy weather. Thirdly, before and after Tomb-Sweeping Day, there were a lot of water vapor in the atmosphere of the south of the Yangtze River, which easily condensed into Mao Mao rain at night. For these reasons, there are many rains during the Qingming Festival. But it may not rain in Tomb-Sweeping Day, but it is a rainy season in Tomb-Sweeping Day.